1. Field of the Invention
The present invention relates to a charge pump, and more particularly, to a charge pump capable of amplifying input voltages and providing corresponding stable output voltages.
2. Description of the Prior Art
Liquid crystal display (LCD) devices are thin flat panel display (FPD) devices characterized in low radiation, small size and low power consumption. LCD devices have gradually replaced traditional cathode ray tube (CRT) devices and have been widely applied in electronic devices such as notebook computers, personal digital assistants (PDA), flat panel televisions, and mobile phones.
Charge pumps are normally used as boosters or voltage multipliers. In LCD devices, a charge pump is often used for boosting the output voltage of a low power source (such as a lithium battery) so as to provide a source driver or a gate driver with a higher working voltage. As well known to those skilled in the art, the polarities of the driving voltages applied to liquid crystal molecules have to be alternated with a certain interval in order to prevent permanent damages of liquid crystal material due to polarization. The amount of current consumed by the source and gate drivers has the maximum value when the polarity of the driving voltage begins to invert. Therefore, the charge pump has the maximum loading at the time of voltage inversion. For an LCD device to function normally and efficiently, the charge pump has to provide a sufficient operational range and efficiency (i.e., voltage gain).
Reference is made to FIG. 1 for a diagram of a prior art constant charge pump 700. The constant charge pump 700 includes a level shifter circuit 27 and a charge exchange control switching circuit 37. The level shifter circuit 27 includes switches SW1-SW4, and the charge exchange control switching circuit 37 includes switches SW5-SW8. VIN and VOUT represent the voltages established at an input end and an output end of the constant charge pump 700, respectively. The level shifter circuit 27 receives clock signals CLK and XCK respectively at a node A1 and a node A2, amplifies the levels of the clock signals CLK and XCK using the switches SW1-SW4, and outputs control signals S1 and S2 with amplified levels respectively at a node B1 and a node B2. The charge exchange control switching circuit 37 controls the switches SW5-SW8 based on the control signals S1 and S2 so that the input voltage VIN can be amplified to the required output voltage VOUT for voltage-boosting. In the prior art constant charge pump 700, the switches SW1, SW2, SW5 and SW6 can include N-type metal-oxide-semiconductor (NMOS) transistors, and the switches SW3, SW4, SW7 and SW8 can include P-type metal-oxide-semiconductor (PMOS) transistors. The prior art constant charge pump 700 can accurately generate the output voltage VOUT by efficiently driving the charge exchange control switching circuit 37 using the control signals S1 and S2 with a full voltage swing provided by the level shifter circuit 27. However, the prior art constant charge pump 700 has the best performance when the amount of loading varies slightly. When applied to designs with large loading variations, the operational efficiency of the constant charge pump 700 greatly attenuates with a small load, and the constant charge pump 700 may not be able to function normally with a large load.
Reference is made to FIG. 2 for a diagram of a prior art capacitive push-pull charge pump 800. The capacitive push-pull charge pump 800 includes a level shifter circuit 28 and a charge exchange control switching circuit 38. The level shifter circuit 28 includes switches SW1, SW2 and capacitors CLS1 and CLS2. The charge exchange control switching circuit 38 includes switches SW3-SW6. VIN and VOUT represent the voltages established at an input end and an output end of the capacitive push-pull charge pump 800, respectively. The level shifter circuit 28 receives clock signals CLK and XCK respectively at a node A1 and a node A2, amplifies the levels of the clock signals CLK and XCK using the switches SW1 and SW2, and outputs control signals S1 and S2 with amplified levels respectively at a node B1 and a node B2. The charge exchange control switching circuit 38 controls the switches SW3-SW6 based on the control signals S1 and S2 so that the input voltage VIN can be amplified to the required output voltage VOUT for voltage-boosting. In the prior art capacitive push-pull charge pump 800, the switches SW1, SW2, SW5 and SW6 can include PMOS transistors, and the switches SW3 and SW4 can include NMOS transistors. The prior art capacitive push-pull charge pump 800 can automatically adjust the amplitude of the control signals S1 and S2 according to the amount of charges provided by the load. Therefore, the amount of charges required for performing charge exchange can be lowered, and the capacitive push-pull charge pump 800 can provide a higher operational efficiency. However, the capacitive push-pull charge pump 800 cannot provide the control signals S1 and S2 with a full voltage swing. As a result, the output voltage of the capacitive push-pull charge pump 800 is less stable and easily varies when the amount of loading changes.